Chemistry Reference
In-Depth Information
is evidence that peracetic acid is formed in the inter-
layers and reacts directly with the substrate there. In
other cases, acetonitrile is a good solvent and the
chemistry appears to arise from protonated H
2
O
2
itself. The interlayer spaces are relatively hydrophilic
and acidic, so olefins are more likely to give diols
than epoxides in this system. However, these spaces
can be modified by pillaring (organic or inorganic)
to give a range of heights and polarities. Some vari-
ation also is possible by templating with non-ionic or
cationic surfactants. Like polyoxometallates, there is
enormous scope for structure manipulation. Combi-
nations of major and minor elements, crystallinities,
etc. can be made, but the potential of this type of cat-
alyst for use with H
2
O
2
has been explored much less
to date. One of their chief attractions compared with
TS-1 is the greater mobility and size of substrates
achievable.
A chromium-pillared zirconium phenylphos-
phonate has been described recently with ca. 2-nm
mesopores [153]. No catalysis results are yet to
hand.
Other smectitic materials include clays, e.g. mont-
morillonite (acid sites), and layered double hydrox-
ides, e.g. gibsonite (base sites). These have been
employed as supports for other catalysts, the former
for cationic species such as bipyridine and triazacy-
clononane complexes, and the latter (which are only
stable over a narrow neutral to alkaline pH range)
for anionic species such as polyoxometallates and
metal-peroxo complexes [161].
centrations and are not very robust to oxidation—
supported or immobilised catalysts have a strictly
limited life. In some cases, such as ligninases, self-
destruction is even a normal part of the mode of
action. This drawback could be overcome by using
whole-cell systems rather than isolated enzymes:
such expertise is not widespread in the chemical
industry at present.
Secondly, the low concentration limit and high
catalyst molecular weight mean that space yield is
poor and recovery/recycling of the enzyme is
awkward. Hence, peroxidases as such are not partic-
ularly attractive as catalysts in industrial oxidation.
This applies particularly to haem-based systems.
Vanadium and molybdenum enzymes are somewhat
more robust but also less active towards substrates of
industrial interest. Peroxidases do have other appli-
cations in synthesis [154].
Of distinct interest, however, are hydrolase (lipase,
esterase) enzymes—not for catalysing H
2
O
2
reactions
directly but for forming more electrophilic inter-
mediates via acylation of H
2
O
2
(or 'esterification'
of acids with H
2
O
2
to give peracids) [155].
These enzymes are much more robust, and one in
particular,
Candida antarctica
lipase, is outstand-
ingly so, such that it has a long lifetime in immo-
bilised form (Novozym
TM
435). A range of peracids
have been generated this way, either from their
acids or from lower alkyl esters. Peracetic acid works
reasonably well but longer chain analogues suit the
enzyme better, especially C
8
and greater. Methyl
oleate reacts in two stages, with the intermediate
peracid epoxidising itself to form 9,10-epoxystearic
acid [156]. Furthermore, acid-sensitive substrates
can be oxidised by a percarbonic acid intermediate
generated from dialkyl (e.g. dimethyl) carbonates
and H
2
O
2
: after reaction, only alcohol and CO
2
are
left [157].
3.5 Enzymes
There has been enormous progress in biotechnology
and in the receptiveness of much of the chemical
industry towards it. Also, oxidoreductase enzymes
are plentiful and many are well characterised and
readily isolated. However, they have significant
drawbacks as industrial oxidation catalysts.
Firstly, most enzymes are not designed to with-
stand significant concentrations (≥1%) of H
2
O
2
because these are not encountered in nature. Hydro-
gen peroxide often is generated naturally from
dioxygen reduction by oxygenases, but invariably
there is co-production of catalase, which destroys the
H
2
O
2
very efficiently without release of other active
oxidants. Peroxidase enzymes do exist, of course,
using H
2
O
2
itself to carry out organic oxidations, but
again these work naturally with small peroxide con-
4 Developments in Catalysed
Oxidations for Chemical Synthesis
In each of the following sections, the aptitude of
H
2
O
2
for the given application will be considered,
major existing uses will be highlighted and chemis-
try available for future exploitation will be re-
viewed. There have been some general surveys of
the catalysed oxidation area, including H
2
O
2
reac-
tions [158-160].
